Christian Muthmuth-software.com
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I understand your business – and build the software that powers it.

Senior Java developer for turnkey new products and modernising established enterprise systems

I build new products turnkey, from idea to operation – AI-assisted, in weeks rather than months. I modernise established systems step by step, while they stay in production. Either way, I get to grips with laws, standards and business processes quickly and thoroughly and deliver lean, maintainable and cost-efficient solutions.

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Since:
software development since 1997, freelancing since 2003
Focus:
Turnkey product development
Legacy modernisation
Spring Boot, Jakarta EE, XÖV, AI
Engagement:
Remote; on site for onboarding and key meetings
Available:
now

Experience you notice in the project

I think in architecture and deliver code – as a complete product from a single source or hands-on within your team, from the new application to the established legacy system.

Method summary
ReturnsMethodDescription
Productbuild(idea)Turnkey product development. From the idea through requirements, architecture, backend and frontend to operation – from a single source, AI-assisted in weeks rather than months, with automated tests at every level.
Applicationextend(requirements)Java backend development. Business applications with Java, Spring Boot and Jakarta EE – well structured, tested and maintainable. Full-stack with React and TypeScript when needed.
Systemmodernise(legacySystem)Legacy modernisation. Step by step rather than big bang: porting legacy applications to Java, replacing file interfaces with events, moving authentication to OAuth2/OIDC – while the system stays in production.
Interfaceintegrate(standard)Interfaces & integration. XÖV and OSCI in German public administration, REST, SOAP, Kafka and payment APIs – built so that data stays consistent even when the other side fails.
Decisionvalidate(measurement, prototype)Prototypes & performance analysis. Backing technology decisions with performance tests, profiling and proofs of concept – rather than opinions.
Teamenable(knowledge)Knowledge transfer & mentoring. Pairing, mob sessions and workshops, so your in-house team can carry the solution forward without external support.
Softwaredeliver(ai, tests)AI-assisted development. Claude Code and Cline for analysing large codebases, planning features and reviews – with automated tests as a safety net.
Working together, the way you need it
ConstructorDescription
ChristianMuth(ServiceContract contract)Time-and-materials engagement, long-term welcome – via agencies or directly.
ChristianMuth(FixedPrice assignment)Clearly scoped tasks such as an application or software component.
ChristianMuth(Remote mode)Almost exclusively remote; on site for onboarding and key meetings.

What clients value

Evidence rather than superlatives – from my projects of recent years.

  • Domain depth, not ticket processing

    Population registers, credit reporting or vehicle diagnostics: I dig into the domain and clarify edge cases with the product owners before they reach the code. When connecting an archive system to a population register, I designed for the archive being unavailable from the start – both systems stay consistent even then.

  • Modernisation, step by step

    I ported business functions of a Uniface legacy application to Java incrementally. For a large insurer, I designed the replacement of nightly file interfaces with Kafka events and showed in a proof of concept how OAuth2/OIDC can replace mainframe authentication.

  • Decisions based on facts

    For the new core application of a major credit bureau, I used performance tests and profiling to prove that the planned stack could not meet the required sub-second response time. This prompted the move to a leaner solution with Camunda.

  • Knowledge transfer that works

    In deliberately mixed teams I help junior colleagues grow: through pairing, mob sessions and talks in cross-team workshops.

  • Fast and turnkey – with quality

    I built a complete sales and point-of-sale system with payment integration and more than 400 automated tests in about six weeks from idea to acceptance, and have been running it ever since – AI-assisted with Claude Code. In client work I use Cline to analyse a codebase of around 48,000 classes.

  • Reliability

    Almost exclusively multi-year engagements: with the municipal IT service provider since 2019, nearly four years with the energy services company, nearly three with the credit bureau and more than two with the car manufacturer.

“Thanks to you, I learned more of practical relevance in this project than in my entire degree.”

@author Junior developer, credit bureau project

Understand first, then build

The best technology is useless if it misses the actual need. That is why every project starts with your business – not with the code.

Project.java
var solution = yourBusiness
// Dig into laws, standards and processes, clarify edge cases early
.understand()
// Back decisions with measurements and prototypes
.validate()
// Lean, tested, in short iterations – AI-assisted
.implement()
// Carry knowledge into the team: pairing, mob sessions, workshops
.handOver();

At home in complex domains

Often regulated environments where business rules and technology must fit precisely.

  • Public administration
  • Credit reporting & financial services
  • Insurance
  • Automotive
  • Energy
  • Public transport

Domain knowledge

  • German population registration law (BMG, register modernisation, self-determination act)
  • XÖV e-government standards (XMeld, XPersonenstand, XAusländer, XPassAusweis, XhD) and OSCI
  • Credit reporting and credit scoring
  • Point of sale and inventory management (FIFO valuation, deposits)

Selected projects

Multi-year engagements in demanding domains – from population registers to vehicle diagnostics. Client names and references available on request.

  • Population register and connected government systems

    2019 – today
    Client:
    Municipal IT service provider, public administration
    Role:
    Senior Java developer

    Ongoing development of population register software used by municipalities of every size up to cities with over a million inhabitants, plus connected state systems for security authorities. Large programme with up to around 60 developers in 6–8 Scrum teams and a codebase of around 48,000 classes.

    • Incrementally ported business functions of the Uniface legacy application to Java, improving them functionally and technically
    • Created, sent via OSCI, received and processed XÖV messages – between registration offices, registry offices, immigration authorities and central registers
    • Built a state-wide facial image retrieval register from scratch in my own Scrum team (2022–2023)
    • Technical lead: archive integration following the transactional outbox pattern, consistent even during temporary outages
    • Interim lead of a team of six developers (2020)
    • Jakarta EE
    • Spring
    • Vaadin
    • XÖV
    • OSCI
    • Oracle
    • Docker
    • Kubernetes
    • Cline

  • Sales and point-of-sale system as a progressive web app

    2026 – today
    Client:
    Getränke Club Salz – a club in my home village
    Role:
    Concept, architecture, development and operations (pro bono)

    Members buy via smartphone and pay online or at a card terminal; the club management controls stock, purchasing, members and revenue. Free of charge including operations – I sponsored the running costs and the payment terminal.

    • Business logic far beyond a web shop: FIFO valuation of goods, deposits as transitory items, refunds that reverse stock and empties
    • SumUp payment integration (online checkout and card terminal) with idempotency and protection against concurrent bookings
    • Around 350 backend tests, component tests and more than 60 end-to-end tests in a real browser
    • About six weeks from idea to acceptance of the payment integration – AI-assisted with Claude Code
    • In production for all members since October 2026
    • Java 25
    • Spring Boot 4
    • PostgreSQL
    • React
    • TypeScript
    • Playwright
    • Claude Code

  • Connecting a legacy application to a modern system landscape

    2026
    Client:
    Large insurance company
    Role:
    Senior Java developer (part-time)

    A Java legacy application manages the portfolio of pension and life insurance contracts. Concepts and prototypes for two sub-projects:

    • Designed and prototyped a Kafka-based interface to replace the nightly file exchange
    • Proof of concept: authentication via OAuth2/OIDC with Microsoft Entra ID instead of CORBA via the mainframe, including seamless Windows sign-on
    • Basis for the decision to introduce Spring into the legacy application
    • Java 17
    • Spring Boot 3
    • Kafka
    • OAuth2/OIDC
    • Entra ID

  • New core application for a credit bureau

    2014 – 2017
    Client:
    Major German credit bureau
    Role:
    Senior Java developer

    Partners such as banks and telecoms retrieve credit reports and scores – synchronously, with response times below one second.

    • Java EE components and independent services, orchestrated first with Oracle SOA Suite, later with Camunda
    • Proved with performance tests and profiling that SOA Suite could not meet the required response time – prompting the switch to Camunda
    • Java EE
    • WebLogic
    • Camunda
    • Oracle
    • Elasticsearch

  • Vehicle diagnostics in an open standard

    2008 – 2010
    Client:
    Car manufacturer
    Role:
    Senior software developer, additionally software architect in 2010

    Diagnostic procedures were written in a proprietary language; the EURO 5 regulation required an open standard.

    • Model-driven transformation into the open standard OTX (ISO 13209) as a plug-in for the in-house authoring application
    • Prototype analysing previously unused diagnostic logs from workshops – the basis for a business case
    • Architecture of a self-learning decision support system
    • Java
    • Eclipse RCP/OSGi
    • EMF
    • GMF
    • UML/MDA
Further engagements
PeriodClientRole and topic
2017 – 2018International software companySoftware architect and backend lead: internal cloud-based event management application, interfaces to enterprise systems, security concept
2011 – 2012International market research companySoftware architect and technical project lead: data analysis platform for international client projects
2005 – 2008Swiss bank and railway companySoftware architect and senior developer: platform for client-specific pricing of complex banking products; performance analysis of a dispatching system
2001 – 2005Energy services company and public broadcasterSubsystem architect and developer: mobile data capture, integration of mainframe and SAP systems, coaching of in-house staff
1997 – 2001IT service providers, telecoms, credit bankSoftware developer: web applications, e-mail and SMS gateways, high-availability platform

Skills

Skills from my engagements since 2017. Older technologies are listed with the respective projects.

Skills summary
AreaExpertAdvanced
LanguagesJava (up to 25), SQL, XML/XSD/XSLTTypeScript, JavaScript, HTML/CSS, Python, Bash
BackendJakarta EE / Java EE (EJB, JPA, JTA, Interceptors), Spring Framework, Spring Boot, JPA/HibernateBean Validation, MapStruct, Lombok, Quartz
Frontend–React, TypeScript, Vite, progressive web apps, Vaadin
DatabasesOracle, PostgreSQLMS SQL Server, SAP HANA, H2, Flyway, Liquibase
Interfaces & integrationXÖV (XMeld, XPersonenstand, XAusländer, XPassAusweis, XhD), REST/JSON, SOAP/WSDL, modernising legacy interfacesOSCI, OpenAPI, Apache Kafka, Microsoft Graph, SAP systems, payment APIs (SumUp, Stripe)
Security–OAuth2/OIDC, Spring Security, Microsoft Entra ID, Kerberos/IWA, JWT, security concepts
ArchitectureLegacy modernisation, reliability of distributed systems (transactional outbox, XA/JTA, retry), performance analysis and profilingArchitecture decision records (ADR), modular architectures
Testing & qualityJUnit, code reviewsTestcontainers, Spring Boot Test, Playwright, Vitest, Testing Library
Build, CI/CD & operationsMaven, GitJenkins, GitLab, GitHub Actions, Docker, Docker Compose, Linux servers, Kubernetes (basics)
AI-assisted developmentClaude Code, ClineContext engineering, prototyping with Claude Design
MethodsScrum, requirements analysis and backlog refinement, knowledge transfer and mentoring–
Domain knowledgePopulation registration and its legislationCredit reporting and scoring, point of sale and inventory
ToolsIntelliJ IDEA, Linux, WindowsEclipse

Christian Muth

I was six when I first sat down at a C64 – and started programming. The fascination of teaching a computer, step by step, what it should do has stayed with me ever since.

I turned that passion into my profession: I have been developing software professionally since 1997 and as a freelancer since 2003. Over the years I have worked with enterprises, mid-sized companies and the public sector, in Germany and Switzerland.

My strength: I get to grips with laws, standards and business processes quickly and thoroughly. I turn this understanding into solutions that work in everyday practice – lean, maintainable and cost-efficient. And I pass my knowledge on, so your team stays self-sufficient in the long run.

I work almost exclusively remotely from Freiensteinau in Hesse, Germany. I am happy to come on site for onboarding and special occasions.

Location:
Freiensteinau, Hesse, Germany
Languages:
German (native)
English (C2)
Dutch (A1)
Methods:
Scrum, pairing, mob programming, code reviews
Security clearance:
willing to undergo German clearance (Ü1/Ü2)
References:
on request

Let’s talk about your project.

Tell me briefly what it is about – I usually reply within one working day.

Send an e-mailkontakt@muth-software.com

Freiensteinau, Germany – remote. Available now.